ESUG 2011, Edinburgh

1. GC in Smalltalk
MarkAndCompactGC
Javier Burroni gera

2. ESUG 2010
Object subclass: #GenerationalGC
generational purgeRoots
collect from
self generational follow: object
followRoots;
followStack; generational moveToOldOrTo: object
to
rescueEphemerons;
fixWeakContainers; generational
flipSpaces fixReferencesOrSetTombstone:
weakContainer
generational addInterrupt

3. ESUG 2011
Object subclass: #VMGarbageCollector
VMGarbageCollector subclass: #GenerationalGC
VMGarbageCollector subclass: #MarkAndCompactGC

4. ESUG 2011
VMGarbageCollector subclass: #MarkAndCompactGC
collect
self old
unseeWellKnownObjects;
followAll;
setNewPositions: oldSpace;
setNewPositions: fromSpace; from
prepareForCompact;
compact: oldSpace;
compact: fromSpace; to
updateOldSpace;
makeRescuedEphemeronsNonWeak;
resetFrom;
decommitSlack;
addInterrupt

5. MarkAndCompactGC
Three phases
Follow (mark)
Set new positions
Compact (move)
collect
self
...
followAll;
setNewPositions: oldSpace;
setNewPositions: fromSpace;
prepareForCompact;
compact: oldSpace;
compact: fromSpace;
...

6. MarkAndCompactGC
followAll
Arena
Precondition: all objects unseen
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
Characters
true
false
nil

7. MarkAndCompactGC
followAll
Precondition: all objects unseen
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
unseeWellKnownObjects
nil _beUnseenInLibrary. Characters
true _beUnseenInLibrary.
false _beUnseenInLibrary.
true
self unseeLibraryObjects;
false
unseeCharacters;
nil
unseeSKernel

8. MarkAndCompactGC
followAll
Arena
Precondition: all objects unseen
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
unseen: “natural” state in Arena

9. MarkAndCompactGC
followAll
Arena
Recognize & Mark all living objects
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
Characters
true
false
nil

10. MarkAndCompactGC
followAll
Arena
Recognize & Mark all living objects
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
followAll
self Characters
followSKernel;
followStack;
true
followExtraRoots;
false
rescueEphemerons;
nil
fixWeakContainers;
followRescuedEphemerons

11. MarkAndCompactGC
followAll
Arena
Recognize & Mark all living objects
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
followsSKernel
self Characters
follow: self sKernel
count: self sKernelSize true
startingAt: 1 false
nil

12. MarkAndCompactGC
follow: root count: size startingAt: base
Arena
Recognize & Mark all living objects
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
arenaIncludes: object
^(oldSpace includes: object) or: [fromSpace includes: object]

13. MarkAndCompactGC
follow: root count: size startingAt: base
Recognize & Mark all living objects
(self arenaIncludes: object)
ifFalse: [
object _hasBeenSeenInLibrary ifFalse: [
object _beSeenInLibrary. Libraries
self follow: object...
Characters
true
false
nil

14. MarkAndCompactGC
follow: root count: size startingAt: base
Arena
Recognize & Mark all living objects
from
old
(self arenaIncludes: object)
ifFalse: [
object _hasBeenSeenInLibrary ifFalse: [
object _beSeenInLibrary.
self follow: object...
ifTrue: [
object _threadWith: reference at: oldIndex.
object _hasBeenSeenInSpace ifFalse: [
" object _beSeenInSpace "
self follow: object...]]

15. MarkAndCompactGC
follow: root count: size startingAt: base
...
B A object _threadWith: reference at: oldIndex.
C
Z
bits
Morris, F. L. 1978. A time-and space-efficient garbage compaction algorithm.
Communications of the ACM. 21, 8, 662-665.

16. MarkAndCompactGC
follow: root count: size startingAt: base
...
B A object _threadWith: reference at: oldIndex.
bits
C
Z
Morris, F. L. 1978. A time-and space-efficient garbage compaction algorithm.
Communications of the ACM. 21, 8, 662-665.

17. MarkAndCompactGC
followAll
Arena
Recognize & Mark all living objects
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
followExtraRoots Characters
self follow: self extraRoots count: 3 startingAt: 1
true
followRescuedEphemerons false
self follow: rescuedEphemerons count:... nil

18. MarkAndCompactGC
followAll
Arena
Recognize & Mark all living objects
from
old
VMGarbageCollector subclass: #MarkAndCompactGC
Libraries
followStack Characters
super followStack
true
rescueEphemerons false
super rescueEphemerons nil
Implemented VMGarbageCollector

19. MarkAndCompactGC
collect
...
setNewPositions: oldSpace;
B A setNewPositions: fromSpace;
bits
C
Z

20. MarkAndCompactGC
collect
...
setNewPositions: oldSpace;
B A setNewPositions: fromSpace;
Z'
C
Z
bits

21. MarkAndCompactGC
collect
...
setNewPositions: oldSpace;
setNewPositions: fromSpace;
setNewPositions: space
self
seenObjectsFrom: space base
to: space nextFree
do: [:object :headerSize | | newPosition nextReference reference headerBits |
newPosition := auxSpace nextFree + headerSize.
reference := object _headerBits _unrotate.
[
headerBits := reference _basicAt: 1.
reference _basicAt: 1 put: newPosition _toObject.
nextReference := headerBits _unrotate.
nextReference _isSmallInteger]
whileFalse: [reference := nextReference].
object _basicAt: -1 put: headerBits; _beSeenInSpace.
auxSpace nextFree: newPosition + object _byteSize]

22. MarkAndCompactGC
collect
Unthread references ...
setNewPositions: oldSpace;
setNewPositions: fromSpace;
setNewPositions: space
self
seenObjectsFrom: space base
to: space nextFree
do: [:object :headerSize | | newPosition nextReference reference headerBits |
newPosition := auxSpace nextFree + headerSize.
reference := object _headerBits _unrotate.
[
headerBits := reference _basicAt: 1.
reference _basicAt: 1 put: newPosition _toObject.
nextReference := headerBits _unrotate.
nextReference _isSmallInteger]
whileFalse: [reference := nextReference].
object _basicAt: -1 put: headerBits; _beSeenInSpace.
auxSpace nextFree: newPosition + object _byteSize]

23. MarkAndCompactGC
collect
...
compact: oldSpace;
B A compact: fromSpace;
Z'
C
Z
bits

24. MarkAndCompactGC
collect
...
compact: oldSpace;
B A compact: fromSpace;
Z'
C
bits

25. MarkAndCompactGC
collect
...
compact: oldSpace;
compact: fromSpace;
compact: space
self objectsFrom: space base to: space nextFree do: [:object |
object _hasBeenSeenInSpace ifTrue: [| moved |
moved := auxSpace shallowCopy: object.
moved _beUnseenInSpace]]

26. MarkAndCompactGC
collect
...
makeRescuedEphemeronsNonWeak;
updateOldSpace;
makeRescuedEphemeronsNonWeak
rescuedEphemerons do: [:ephemeron | ephemeron _haveNoWeaks]
updateOldSpace
oldSpace loadFrom: auxSpace

27. MarkAndCompactGC
collect
Free resources ...
decommitSlack;
decommitSlack
| limit delta |
limit := self nextFree + 16rFFF bitAnd: -16r1000.
delta := self commitedLimit - limit.
delta < 0 ifTrue: [^self grow].
delta > 0 ifTrue: [
limit _decommit: delta.
self commitedLimit: limit]

28. MarkAndCompactGC
collect
Free resources ...
decommitSlack;
assembleDecommit
...
return := assembler
pushConstant: 16r4000;
pushArg;
pushR;
callTo: 'VirtualFree' from: 'KERNEL32.DLL';
convertToNative;
shortJump

29. Stop
We have a Smalltalk Scavenger

30. Stop
We have a Smalltalk Scavenger
Written in Smalltalk

31. Stop
We have a Smalltalk Scavenger
Written in Smalltalk
Using objects

32. Stop
We have a Smalltalk Scavenger
Written in Smalltalk
Using objects
Instantiating objects

33. Stop
We have a Smalltalk Scavenger
Written in Smalltalk
Using objects
Instantiating objects
How can it work?

34. Stop
We have a Smalltalk Scavenger
Written in Smalltalk
Using objects
Instantiating objects
How can it work?
Object Closure

35. Object Closure
What happens in VegasGC stays in VegasGC
Self contained objects graph
Created objects do not live after GC
No message new in our code
Created objects:
Block Closures
Environment Contexts
Arrays
To interface with the Host VM

36. Object Closure
BlockClosure
makeRescuedEphemeronsNonWeak
rescuedEphemerons do: [:ephemeron | ephemeron _haveNoWeaks]
Environment context
compact: space
| moved |
self objectsFrom: space base to: space nextFree do: [:object |
object _hasBeenSeenInSpace ifTrue: [
moved := auxSpace shallowCopy: object]
The VM will instantiate both at end of young space

37. Object Closure
BlockClosure
makeRescuedEphemeronsNonWeak
rescuedEphemerons do: [:ephemeron | ephemeron _haveNoWeaks]
Environment context
compact: space
| moved |
self objectsFrom: space base to: space nextFree do: [:object |
object _hasBeenSeenInSpace ifTrue: [
moved := auxSpace shallowCopy: object]
The VM will instantiate both at end of young space
we only scan up to space's size when the GC starts

38. Object Closure
Host VM interface arrays
allocateArrays
rememberSet on: oldSpace; emptyReserving: 16r100.
literalsReferences emptyReserving: 16r200.
classCheckReferences emptyReserving: 16r100.
nativizedMethods emptyReserving: 16r100.
self
allocateWeakContainersArray;
allocateEphemeronsArray;
forgetNativeArrays
allocated at end of oldSpace, just before returning

39. Object Closure
Host VM interface arrays
at: index
^contents _basicAt: index + 1
at: index put: value
^contents _basicAt: index + 1 put: value
use _primitives instead of primitives

40. Demo

41. [Audience hasQuestions] whileTrue: [
self answer: Audience nextQuestion].
Audience do: [:you | self thank: you].
self returnTo: Audience